Electric and magnetic forces: electromagnets, motors and generators
الأسبوع 9 · الدرس الثالث «القوى الكهربية والمغناطيسية»: التأثير المغناطيسي للتيار والمغناطيس الكهربي وقوته والقوة المؤثرة في سلك يمر به تيار والمحرك الكهربي وعزم الازدواج والمولد الكهربي · 3 دروس
2
A force on a wire: the electric motor
Science Prep 3 Term 1, Unit 2: pp. 54–63
GoalExplain the force on a current-carrying wire in a magnetic field and how a DC motor uses it to turn.
New words
magnetic force · the force on a current-carrying wire placed in a magnetic field, at right angles to the wire and to the fieldtorque (couple) · the turning effect of two equal opposite forces acting along different linescommutator · a split metal cylinder in a DC motor that reverses the current in the coil every half turn so that it keeps turning the same way
Explanation
A straight wire that carries a current and lies across a magnetic field feels a magnetic force at right angles to both, which may move it. A simple DC motor uses this idea. A coil of insulated copper wire can turn between the poles of a permanent magnet. Its ends are joined to the source through a split-ring commutator and two graphite brushes. The current goes up one long side of the coil and down the other, so the forces on the two sides are equal, opposite and along different lines. Such a pair of forces is a couple, and it turns the coil. The torque is larger with more current, more turns, a larger coil area and a stronger or more numerous magnet. Reversing the current or swapping the poles of the magnet reverses the rotation. A motor changes electrical energy into kinetic energy, for fans, blenders, washing machines, electric cars and the metro, but it is not 100 % efficient: some energy becomes heat. A coil joined to a battery for long gets hot. A drill-screwdriver changes the motor’s speed for different jobs.
Change
Effect on a motor
more current
larger torque
more turns
larger torque
stronger magnet
larger torque
reverse the current or swap the poles
rotation reversed
Worked examples
A motor takes 20 J of electrical energy and gives 16 J of useful kinetic energy. The rest, 20 J − 16 J = 4 J, becomes heat. The energy is all accounted for, and the motor is not 100 % efficient.
A toy motor spins clockwise on one cell. The two wires are swapped on the cell and it spins anticlockwise. Reversing the current reverses the force on each side of the coil, so the coil turns the other way.
Why does a blender motor run hot after a long time? Because some electrical energy becomes heat inside the coil, instead of kinetic energy. This is why motors are made to rest, and why a grown-up must never hold a motor stuck while on.
Class activity
Toy motor, with a grown-up. Connect a small toy DC motor to one 1.5 V cell and watch it spin. Swap the two connections and describe what changes. Never hold the shaft while connected, and do not leave it on for long. Draw the main parts from memory: coil, magnet poles, split-ring commutator, brushes, source. Draw the two forces on the coil sides as equal arrows pointing opposite ways.
Practice
1
In which direction is the force on a current-carrying wire in a magnetic field?
At right angles to the wire and to the field.
2
What is the torque on the coil made of?
Two equal and opposite forces on the two long sides, along different lines.
3
Name two ways to make a motor turn faster or with more torque.
For example, more current or a stronger magnet (more turns or a larger coil also help).
4
A motor takes 50 J and gives 42 J of kinetic energy. How much becomes heat?
50 J − 42 J = 8 J.
5
What energy change takes place in a motor?
Electrical energy to kinetic energy (with some heat).
